Molding grade copolymers and process for preparing the same



Sept. 18, 1962 s. c. LASHUA 3,054,783

MOLDING GRADE COPOLYMERS AND PROCESS FOR PREPARING THE SAME Filed Oct.22, 1958 14/,0/10 Me/Ay/ .5 {gran e INVENTOR.

6/)6/1770/7 G Lars/20a 3,054,733 MOLDING GRADE COPOLYMERS AND PROCESSFOR PREPARING THE SAME Sherman C. Lashua, Midland, Mich, assignor to TheDow Chemical Company, Midland, Mich, a corporation of Delaware FiledOct. 22, 1958, Ser. No. 769,031 9 Claims. (Cl. 26080.5)

This invention relates to improved thermoplastic copolymers useful inthermal molding procedures. More particularly it relates to suchcopolymers exhibiting improved physical properties. The inventionadditionally contemplates the process by which these copolymers areprepared.

Polystyrene and other homopolymers of monovinyl aromatic monomers areamong the most widely used materials in preparing articles by thermalmolding operations. The articles so prepared are characterized byexceptional properties that make the articles well adapted for many of aWide variety of uses. However, those homopolymers are not withoutdisadvantages. One of the principal disadvantages involves therheological properties of the polymers. In order to obtain the requisitetensile and impact strength, elongation and similar properties of aminimum order for general acceptability, it has been necessary to employa polymer of certain minimum average molecular Weight. When it isdesired to obtain enhanced properties in the polymer product, it is thennecessary to increase its average molecular weight. However, as theaverage molecular weight of such homopolymers is increased, thetemperatures and/ or pressures required for their thermal fabricationare, accordingly, proportionately increased. Thus, in preparing suchhomopolymers, it has always been necessary to reach a compromise in themolecular weight between that which gives optimum physical properties ofthe copolymer and that which permits the most convenient fabricationtemperatures and pressures.

In order to circumvent such problems, attempts have been made tocopolymerize styrene with a Wide variety of comonomers, such asbutadiene, acrylonitrile, alkyl acrylates, and the like in order tomodify some property of homopolystyrene or for some other reason. Noneof those copolymer products so obtained, however, has overcome thedifficulties due to the physical properties-fabrication temperaturecontradiction which is believed to be caused by the rheologicalproperties inherent in the linear or branched copolymer productsgenerally obtained by conventional procedures.

copolymerization also introduces other vexatious complications. Forexample, the products of conventional copolymerizations are generallyobtained as conglomerations, mixtures, or blends of compositionalvariants resulting from the differences in copolymerizability of variousmonomers. The substituents attached to the terminal carbon atoms of theolefinic unsaturation determine to a great extent the ability of aparticular monomer to polymerize or to copolymerize. They also influencethe rate at which a particular monomer wil'l copolymerize with another.

Thus, when two difierent monomers are intermixed and caused tocopolymerize one of the monomers will ordinarily be found to havegreater polymerizability. As a consequence, the more polymerizablemonomer in a mixture of monomers may polymerize with itself in theinitial portion of the reaction to form a homopolymer until itsconcentration in the reaction mass has dropped appreciably. Only thenwill the less polymerizable monomer be able to actually copolymerizewith the faster monomer. The actual copolymerization further depletesthe relative concentration of the faster monomer and proceeds only forthe limited time until the point is reached where substantially only thehomopolymer of the slower monomer is capable of being formed. The resultis a polymeric mixture of homopolymers and copolymers.

As can be readily appreciated, the make up of the conglomeration willusually be found to vary with the initial ratio of monomers present inthe reaction mass, the conditions of polymerization, the purity ofstarting materials, and many other factors that Will be apparent tothose skilled in the art. Reproducibility of the conglomeration frombatch to batch is very difficult, if not entirely impossible. It isfrequently necessary to blend batches of such inconsistent polymers toprovide products having the desired properties. Such techniques, ofcourse, are costly and time consuming.

Many of the prior attempts to minimize compositional heterogeneity ofcopolymerization products have involved continuous polymerizationprocesses. In most of those processes, it has been attempted to adjustthe feed stock to that which would give a particular copolymer productdesired. While suitable products can be made by following the priorprocesses, such processes, almost With out exception, involve delicatecontrol and, despite meticulous execution, still leave room forconsiderable improvement in compositional heterogeneity. Thus, it wouldbe desirable to have a process which would provide compositionalhomogeneity and easy control of copolymerization.

Among the copolymers which have appeared to have unique properties foruse inmolding are those of the vinyl aromatic monomers, such as styrene,with the alphabeta unsaturated carboxylic acids, such as acrylic andmethacrylic acids. Even when prepared by the conventional batchwiseprocesses, the copolymers exhibit improved physical properties, such asincreased heat distortion temperatures over either of the homopolymers.However, the monomers involved in the preparation of such productsresult in so much heterogeneity in the product that it is difiicult toreproduce the properties of the product from batch to batch. Also, inthe batchwise processes, the copolymers tended to cross-link at highconversions. When the copolymers are prepared by the aforementionedcontinuous processes, the properties of the resulting product aresomewhat improved, but the copolymers tend to exhibit a thermalinstability, as evidenced by a progressive decrease in melt viscosityupon continuous exposure to elevated temperatures. Significant evidencethat the heretofore available processes result in heterogeneouscompositions resides in the appearance of molded sections prepared fromthe copolymer products of such processes. Such sections are usuallytranslucent or opaque. Thus, it would be particularly desirable to havean improved process for preparing styrene-acrylic and the likecopolymers, so that their full potential as molding materials could beeasily realized and readily achieved.

In copending US. patent application Serial No. 702,376, filed December12, 1957, there is described a process for copolymerizing styrene andacrylic acids to achieve a polymeric product of improved homogeneity.That process involves a continuous copolymerization of the comonomersunder equilibrium. The products of that process exhibit exceptionallyhigh heat distortion temperatures, and other properties. However, underthermal influence they tend to shrink, causing molding difiiculties.Also, many such copolymers craze under molding conditions.

The provision of an improved class of terpolymers from vinyl aromaticmonomers and crylic monomers is the principal object of this invention.A further object is the provision of such a class of terpolymers whichexhibit practically no tendency to shrink upon thermal exposure.

A still further object is to provide such a class of terpolymers havingimproved compositional homogeneity, thermal stability and clarity overprior known terpolymers of similar composition.

Still another object is the provision of an improved continuouspolymerization process for preparing these terpolymers.

The above and related objects are accomplished with a terpolymercomposed of a monovinylaromatic monomer, alpha-methylstyrene andmethacrylic acid in proportions as will be described later. The objectsare further realized by means of the process wherein a homogeneousmonomeric material of the above composition and falling within theproportions as will be set forth is prepared and heated together withnot more than about 60 percent of the terpolymer product resultingtherefrom at an elevated temperature while maintaining the proportionsof said monomeric material and said terpolymer product substantiallyconstant by feeding additional amounts of said monomeric material to themixture at substantially the same rate at which it is polymerized Whilewithdrawing said terpolymer product at about the same rate at which saidmonomeric material is fed to the mixture.

The terpolymers contemplated by the invention are those containingrecurring units of polymerized monovinyl aromatic monomers of thebenzene series as will be described and also containing in the samepolymer rene or or-tho-para-dichlorostyrene, or comonomeric mixtures ofor styrene with any of the above-named compounds may also be used. Thus,the term monovinyl aromatic monomer as used herein is intended toinclude the compounds having a vinyl radical directly attached to acarbon atom of an aromatic nucleus containing from 6 to 10 carbon atoms.

The invention requires the use of methacrylic acid. As

'will be shown acrylic acid even in the expressed proportions does notresult in the desired properties. It has also been found that crotonicacid and itaconic acid are generally not satisfactorily operable in theprocess for the primary reason that each is insoluble in the mono vinylaromatic monomer. Solubility is a prerequisite for successful operation,since only when such condition is achieved is it possible to achieve ahomogeneous monomeric material and consequently the desired polymerproducts. In addition acrylate esters, such as the alkyl acrylates whenused as the sole acrylic monomer, do not provide the outstandingproperties desired, herein. However, small amounts of alkyl acrylatesand alkyl methacrylates may be used if desired to form a quaternarypolymer.

Excellent molding grade copolymers are obtained when the monovinylaromatic monomer is employed in the homogeneous monomeric material in aconcentration of from about 55 to about 97 percent of the total weightof monomeric material with the remainder being made up to from about 1to 40 percent of alpha-methylstyrene and from 2 to 15 percent of theacrylic monomer all as further defined in the annexed drawing, FIG. 2,as areas A-l-B-l-C. Advantageously the monomeric composition may consistof from about 55 to 90 percent by weight of the monovinyl aromaticmonomer from about 1 to 40 of alpha-methyl-styrene, and from about 3 to15 percent by weight of methacrylic acid (areas A+B). A preferred rangeof monomeric composition consists of from about 55 to 75 percent byweight of the monovinyl aromatic monomer from about 15 to 40 percent ofalpha-methyl- ''styrene and from about 5 to percent of methacrylic acid(area A). There is no significant advance in copolymeric properties whenthe base polymer is prepared from a monomeric material containing lessthan about two percent of methacrylic acid. Terpolymers prepared frommonomeric materials composed of appreciably more than about 15 percentof methacrylic acid are difficult and expensive to prepare, have asignificantly reduced polymerization rate, and are so cross-linked as tobe of little utility as molding materials. In addition the desiredproperties of non-shrinkage, non-crazing and the like are attained onlywhen the alpha-methylstyrene is employed in the stated range. Anyadvantage obtained from those terpolymers of composition outside of theclaimed monomeric proportions by way of their inherent properties is notproportional to that obtained when other than the expressed amounts ofmonomers are used. In addition such advantage, even when achieved, seemsto be oif-set by an undesirable reduction in some other property of theproduct, and by the increased corrosive tendencies of the monomers whenthe acrylic monomer is in excess, which increase the difficultiesincurred in successfully accomplishing the polymerization.

When employed in the expressed proportions and polymerized underequilibrium conditions these monomers result in polymers having acomposition of from about 60 to percent styrene, from about 1 to 25percent alpha-methylstyrene and from about 4 to 25 percent methacrylicacid. The advantageous range gives a polymeric product having from about60 to 85 percent styrene, from 1 to 25 percent alpha-methylstyrene, andfrom about 5 to 25 percent methacrylic acid. The preferred monomericproportions result in a terpolymer of from about 60 to 75 percentstyrene, from about 12. to 25 percent alpha-methylstyrene and from about8 to 17 percent methacrylic acid.

The copolymers of this invention are preferably made by the processcomprising heating a homogeneous mixture of the monomeric material andthe polymeric product thereof in bulk, i.e., in the substantial absenceof other polymerizable substances, or insoluble dispersants, or in thepresence of solvents for the monomers and polymer at a pressuresufiicient to preserve the liquid state. The polymerization temperatureis maintained between about C. and about C. The monomeric materials arefed into said mixture at a rate and in a proportion such as to maintainin said mixture a constant equilibrium ratio of the monomeric materialsto each other and to maintain the proportion of terpolymer in themixture constant at not more than about 60 percent by weight.

These conditions are readily attained by feeding continuously, and atsteady rates, the polymerizable monomeric materials to the polymerizingsystem in a polymerization zone which is maintained within the indicatedrange of temperature while continuously withdrawing from saidpolymerization zone a portion of the polymerizing system orpolymerization mass at a rate correspond ing to the volumetric feed rateof monomeric materials. When equilibrium is attained in such a system,the ratio of monomeric materials to one another in the polymerizationzone, the concentration of polymer in the polymerizing system, and thechemical composition of the polymer product are generally found to besubstantially constant. It should be apparent that the relativeproportions in which the monomeric materials are chemically combined inthe terpolymer are not necessarily the same as the relative proportionsof the monomeric materials in the homogeneous mixture from which thetel-polymer was formed. The proportions of monomeric materials which arenecessary in a feed mixture in order to produce a terpolymer having aparticular empirical structure may be determined from the pertinentreactivity ratios or, if such ratios are not available, the suitableproportions may be found by simple preliminary tests. As a practicalmatter, once the equilibrium has been established, it is mostconveniently continued by maintaining a heat balance on the systemthrough adjustment of the feed rates of monomeric material and rate ofwithdrawal of the mixture of polymer product and unpolymerized monomers.

For successful operation of the process, it is essential that all stepsup to the withdrawal of the portion of the polymerizing system beconducted in the substantial absence of iron. Best results are obtainedwhen all iron is effectively excluded from the polymerization system.Thus the use of apparatus constructed of ordinary iron and steel isprohibited. Elemental or ionic iron, even in a concentration of partsper million or less, has such a retardant effect on the rate ofpolymerization as to preclude acceptable polymerization rates. Even thecommon stainless steels, when used as materials of construction, retardthe rate of polymerization somewhat, although not prohibitively. Afurther disadvantage of the presence of iron is the discoloration of theresultant polymeric product and also the increase in the thermalinstability of the product. For the latter reasons it is preferred toexclude iron from all steps of the process including those relating towithdrawal of the polymeric product and subsequent processing steps,such as devolatilization, grinding and the like. Materials ofconstruction, such as non-ferrous alloys, nickel, glass-lined steel, andthe like, are well-adapted for use in carrying out the process and, whenso carried out, the polymeric product has maximum and reproducibleproperties.

When operating in accordance with the preferred process of thisinvention at the expressed temperature range of from about 130 C. to 160C., it has been found that the pressures encountered are in the range offrom about 25 to 75 pounds per square inch gauge. Those pressures willmaintain the monomeric materials in the liquid state.

Because the preferred process operates under equilibrium conditions, itis preferred to conduct it so that the polymerization systemcontinuously contains less than about 60 percent of solids. By 60percent or less solids content in the polymerization system is meantthat the polymerization mixture or mass contains not more than 60percent by weight of polymer product. When the process is operated suchthat the polymerization system contains substantially more than 60percent solids, the desirable conditions of equilibrium are found to bemore difiicult to achieve and maintain and the polymer product is lesssubject to precise control. Additionally when higher solidsconcentrations are involved, practical problems such as uniform heattransfer and pumping of the viscous polymerizing system, become quiteserious. Although solids of less than 60 percent may be utilized as theequilibrium, the full potential capacity of the apparatus is notrealized and the subsequent devolatilization is prolonged. In order tomaintain an equilibrium it follows that the percent solids at any momentwill be greater than the percent conversion of monomer to polymer. Inpractice, it is preferred to adjust the rates of feed and withdrawal soas to operate at very low conversions of about 1 to 5 percent or less.That results in greater compositional homogeneity.

Although minor amounts of the usual free radical polymerizationcatalysts may be employed in the process, they are not essential to itssuccessful operation. Accordingly, it is preferred to operate withoutany added catalyst. As will be later described, however, catalysts maybe employed in adjusting the process to provide the copolymer productshaving the desired viscosity.

The polymer product can be isolated from the unpolymerized components ofthe polymerization mixtures by any of the known methods, such as byprecipitation in a non-solvent or by devolatilization. It is preferredto devolatilize the mixture mechanically by heating the mixture underreduced pressure whereby the volatile unpolymerized components arevaporized leaving the polymer product as a residue. If immediatedevolatilization is impractical upon withdrawal of the mixture from thepolymerization zone, the mixture may advantageously be chilled to stopor to retard subsequent polymerization to the greatest possible extent.If polymerization is allowed to continue beyond withdrawal of thepolymerization system from the polymerization zone, it generally doesnot proceed according to and under the necessary conditions of thepresent process but will ordinarily follow the usual batchwisepolymerization mechanism and kinetics. The results in such instanceswill effectively be a blend of the desired polymeric product of thisinvention and of the conventional batch-polymerized product. Such ablend is, of course, likely to be obtained with the aforementioneddisadvantages of compositional heterogeneity, thermal instability,translucence, gels due to cross-linking, and others. As a generallyinflexible rule, only the prod ucts resulting from the present processare found to exhibit the indicated desirable properties.

To afford further illustration, a preferred embodiment of an apparatuswhich is well-adapted for the manipulative steps of the preferredprocess will be described. It should be understood that the invention isnot limited to any particular apparatus and that known mechanicalequivalents may be substituted in the preferred embodiment withoutdeparting from the spirit and scope of the invention. In the drawings:

FIG. 1 is a schematic plan of the apparatus and FIG. 2 is the previouslyreferred to ternary composition diagram.

In the preferred embodiment there is provided a loop 10 of pipe forminga torus, herinafter referred to as coil 10. A recirculating pump isprovided in the coil 10 to recirculate the polymerizing mixture throughthe coil. An inlet pipe 12 is attached to the coil 10, preferably nearthe recirculating pump 11. The pipe 12 causes dispersion of monomericmaterials within the polymerizing system as rapidly as possible so as tomaintain the polymerization equilibrium. Fitted into inlet pipe 12 is ametering pump 13 for metering a homogeneous monomeric material into thecoil 10. The inlet pipe 12 is connected to conventional inventory tank(not shown). At a point on the coil 10 remote from the inlet pipe 12 isan outlet pipe 14 for withdrawing a portion of the polymerizationsystem. Fitted into the outlet pipe 14 is a metering pump 15 forwithdrawing a metered portion of the polymerizing materials. The outletpipe 14 is connected to a conventional dcvolatilizer such as thatdescribed in U.S. 2,804,920, issued September 3, 1957 to C. C. Perkinsor other similar devolatilizing apparatus fines shown) which is adaptedfor handling viscous A temperature regulating control 16 is installed inthe system. A thermocouple 17 is inserted in a Well in coil 10 andconnected to control 16. A valve 18 is fitted into inlet line 12 betweenmetering pump 13 and coil 10. The control is adjusted so that anincrease of temperature within the coil 10 will open the valve 18 andcause more monomer to be introduced into the polymerizing system.

A pressure regulating control 19 is also inserted into the system.Connected to the control 19 is a valve 20 which is fitted into theoutlet line 14 between the coil 10 and the metering pump 15. The control19 is also connected to inlet line 12 at the point designated by thenumeral 21. The control 19 is adjusted so that an increase in pressurein the inlet line 12 will open the valve 20. This causes a portion ofthe polymerizing system to be withdrawn.

The use of the temperature regulating control 16 and the pressureregulating control 19 permits the process to be subject to automaticcontrol. Once the coil 10 is hydrostatically filled and equilibrium hasbeen established, a slight increase in temperature within thecoil 10, asmay be caused by the exothermic polymerization 7 reaction, opens thevalve 18. In this way, fresh, relatively cold monomer is automaticallyintroduced to the polymerization system to lower its temperautre.Simultaneously, the valve 20 opens to allow the discharge of an equalvolume of monomer-copolymer mixture.

If desired a part of the coil 10 may have a jacket 24 through which heatexchanging fluids may be passed by means of inlet 22 and outlet 23. Suchmeans allows the polymerization to be controlled at any desired solids.

The copolymer products produced in accordance with the present processare thermoplastic and capable of being formed by conventional thermalfabrication methods, such as compression and injection molding, intouseful articles of manufacture. The products exhibit improvedproperties, particularly with regard to thermal stability, shrinkageresistance and to the transparency of the molded articles over thecopolymers and products of closely related monomeric composition. Inaddition, the copolymers of this invention are free of gels and the likewhich are commonly present when a copolymer is partially cross-linked,highly branched, or of highly heterogeneous composition. The lack ofsuch defects is of paramount importance in achieving commerciallysuccessful moldings, particularly transparent moldings. Still further,the improved properties of the copolymer products obtained by practiceof the present invention are more easily and consistently reproduciblethan when the prior processes are employed for their preparation.

It is believed that the aforementioned improvements in the polymericproduct prepared in accordance with the preferred process are the resultof a more regular and uniform molecular architecture. of the copolymerthan that of terpolymers prepared by the prior known processes. In theprior processes the resulting polymeric products had highly irregular,non-uniform, heterogeneous architecture which depended upon the amountof premixing of the monomers and the relative chemical reactivities ofthe monomers. In the process of this invention, however, it is believedthat there are produced terpolymers whose architecture very closelyresembles the theoretical recurring groups that could be drawn from anexamination of the starting monomeric mixtures.

Because of the molecular architecture and composition, the polymerproducts obtainable by practice of the present invention generallyexhibit a rheological behavior which is exceptionally well-adapted forthermal fabrication. It is believed that the carboxylic groups arecapable of such an alignment or orientation that they hydrogen bond withone another. Thus, at ordinary temperatures, the terpolymers exhibitstrengths approaching those of copolymers which are cross-linked byprimary valence bonds. However, at elevated temperatures the terpolymersof the present invention exhibit the fluidity or plasticity which isusually associated with linear polymers. As a result, they can be easilyformed or shaped into practically any desired article. Upon cooling froman elevated temperature, the pseudo-crosslinking of the presentterpolymers caused by the hydrogen bonding is generally observed torecur. This behavior likewise results in higher heat distortiontemperature for the copolymers. However, although the heat distortiontemperature is increased appreciably, there is only a slight elevationof the temperature required to fabricate the copolymers thermally. Thisbehavior is in contrast to the prior experience in the polymerizationart, where a modification which would cause an increase in the heatdistortion temperature would necessarily result in a proportionalincrease in the fabrication temperature. This effect is of readilyapparent commercial significance.

In addition because of their monomeric composition these terpolymersexhibit significantly less tendency to shrink upon thermal exposure thanthose terpolymers outside of the expressed compositional range or ofthose binary polymers of styrene and acrylic or methacrylic acid.

For use in molding applications the copolymers should have a solutionviscosity of from 4 to 20 centipoises, and preferably from 5 to 10centipoises, as measured from a 10 percent by weight solution ofterpolymer in methyl ethyl ketone at room temperature. The solutionviscosity is a parameter of the molecular weight for any giventerpolymer wherein the polymer chains are of similar spatialconfiguration. By that is meant that all of the chains are linear orbranched or cross-linked. Thus, for a given terpolymer an increase insolution viscosity indicates an increase in molecular weight. In anymolding operation there must be a compromise between the physicalproperties of the molded article and the melt viscosity of the polymerfrom which the article is molded. Both the physical properties and themelt viscosity are dependent, in the case of linear polymers, upon themolecular weight of the polymer. The above-stated range of solutionviscosities provides copolymers having the optimum prop erties andworkability and is accordingly preferred. The desired range is attainedby operating within the aforementioned temperature range of from about130 C. to 160 C. Within the expressed range it may be desirable toemploy minor amounts of the conventional free radical catalyst toachieve the desired viscosity at the temperature which gives optimumpolymerization rates.

Although the terpolymer products of the present invention may, per se,be formed into useful molded articles, it is most common in the polymerart to employ formulations consisting of many ingredients. Thus, thecopolymers may be dyed, pigmented, filled, plasticized or formulated inany conventional or desired manner.

The following examples are offered for purposes of illustrating andexemplifying the operation of the process and advantages of thecopolymers. In the examples, all parts and percentages are by weight.

EXAMPLE I A polymerization reactor was constructed from a loop of coiledstandard 1 inch pipe about inches in length. The ends of the pipe wereconnected to a pump for rapidly circulating the contents of the coil.Means were provided for continuously feeding, at a measured uniformrate, a homogeneous mixture of liquid monomeric materials. Means werealso provided for maintaining the temperature of the coil at atemperature of from about C. to about 160 C. At a point in the coilremote from the point of introduction of the feed mixture there wasprovided means for continuously withdrawing a portion of thepolymerization mixture at the same rate as the rate of feed into thecoil and under conditions that the contents of the coil were maintainedin the liquid state. The withdrawn portion of the polymerization mixturewas fed into a devolatilizer where the mixture was heated under reducedpressure and the unpolymerized monomers vaporized leaving the copolymerproduct as a residue containing approximately 1 percent of volatilematerial.

Using the above apparatus, copolymers were prepared from styrene andmethacrylic acid; from styrene and alpha-methylstyrene; and also fromstyrene, alpha-methylstyrene and methacrylic acid, each polymer invarious compositional ratios. The temperature of the coil was maintainedthroughout each of the polymerizations at C. The monomers were firstintermixed in the indicated proportions, then fed into the coil, whereinthey were allowed to polymerize to about 50 percent conversion, afterwhich a portion of the copolymer-monomer mixture was continuouslywithdrawn at the same rate at which the monomers were being introduced.After equilibrium was established, the withdrawn polymermonomer portionswere devolatilized.

Heat distortion temperatures were determined. The term, heat distortiontemperature, for present purposes, should be construed as thattemperature which results when a sample of specified dimensions isloaded in a 9 prescribed manner and heated at a fixed rate until thesample is deformed to a stated value. Such a test method is described inA.S.T.M. Tentative Method of Test for Heat Distortion Temperature ofPlastics (D-648-44T),

. in the 1944 Book of A.S.T.M. Standards, Part III, page 1627. Amodification of that method is described in A.S.T.M Bulletin No. 134,May 1945. In the modified test, a compression molded sample is cut intoa test specimen having dimensions of 1.75 by 0.5 by 0.375 inch. The loadis applied until a given deformation is obtained. The temperature atwhich that deformation results is considered to be the heat distortiontemperature.

Table I MONOMER FEED COMPOSITION (PERCENT) The results shown in Table Iindicate the synergistic effect of the alpha-methylstyrene and acidiccomonomers on the heat distortion temperature. It should be expectedthat the heat distortion temperature of the ternary polymers would beintermediate to that of the binaries with styrene. As shown such is notthe case but rather the heat distortion temperature of the ternary issignificantly higher than either binary.

EXAMPLE H Compositions were prepared as in Example I using apolymerization temperature of 140 C. The polymeric products wereinjection molded into test bars. The test bars were exposed to 15 poundsper square inch steam for about 90 minutes in a pressure cooker. Thiscompares to a temperature of about 120 C. The difference in length afterexposure divided by the original length times 100 gave the percentshrinkage. The results are shown in Table II wherein MAA representsmethacrylic acid.

HMMNDOOD CONUIQOO tOWOGOIM 9999. OOQOOQ By way of further comparisonsimilar compositions were prepared using acrylic acid in place ofmethacrylic acid. Test bars were prepared and compared with those of 10Table II as to sttinkage in steam. Heat distortion temperatures werealso run as in Example I and compared with the results there shown.These results are as follows:

Table III COMPOSITION Alpha- Percent HD Styrene methyl- AA Shrink- Temp.

EXAMPLE III Compositions were prepared in the manner described inExample I. The monomeric feed mixtures were used in accordance with thefollowing recipes.

FEED COMPOSITION Alphamethylstyrene MAA womcncom Test bars were preparedfrom each polymeric product. None of the test bars exhibited anysignificant shrinkage after immersion in boiling water for six hours.

EXAMPLE IV (MOLDING TEMPERATURES) Several samples of varying polymericcomposition as shown in Table IV were prepared according to the methoddescribed in Example I with a polymerization temperature of 140 C. Afterdevolatilization the polymeric materials were checked for heatdistortion temperature and for fabrication temperature. The fabricationtemperature, for purposes of this example, is that temperature which is25 F. above the temperature at which a mold will just fill when a rampressure of 10,000 poundsper square inch is applied. For this example, aone ounce standard Watson-Stillman injection molding machine was used.The results of this test program are listed in Table IV.

Table IV COMPOSITION Heat Dis- Fabrica- Styrene MAA a-Methyltortion tionstyrene Tempera- Temp.

ture 0.) F.)

For Comparison This Invention The significance of these results is ofconsiderable comnrercial importance. The comparative compoundsillustrate the dilemma that has always been present in the past. Thus,Compositions having high heat distortion temperatures could be preparedbut only with a corre sponding increase in the practical temperature atwhich the compositions may be fabricated. High fabrication temperaturesare costly to attain and maint ain and increase the possibility ofcompositional de terioration and degradation and present difficultrheological problems. In contrast with the compositions of the presentinvention, it is possible to prepare articles having exceptionaldimensional stability which can be fiabricated on conventional apparatusat the usual temperatures.

What is claimed is:

1. A process for preparing thermoplastic copolymers of molding gradecomprising, as continuous steps (1) the preparation of a homogeneousmonomeric material consisting of essentially of from about 55 to 97percent by weight of a monovinyl aromatic monomer of the benzene seriescontaining only hydrogen substitution on the vinyl group and from about1 to 40 percent by weight of said monomeric material ofalpha-methylstyrene and from about 2 to 15 percent by weight ofmethacryclic acid, (2) the exposure in a polymerization zone of amixture of said homogeneous monomeric material and the copolymer productof polymerization thereof in the liquid state to a polymerizationtemperature between about 130 C. and about 160 C., the proportion ofsaid copolymer product being not more than about 60 percent by weight ofsaid mixture, (3) the maintenance of the proportions of said monomericmaterial and said copolymer product in said mixture substantiallyconstant by feeding additional of said monomeric material to the mixturewhile withdrawing said mixture of copolymer product and unpolymerizedmonomers from said zone at about the same volumetric rate at which saidmonomeric material is being fed to said mixture, all of said steps beingcarried out in the substantial absence of iron, and finally (4)isolating said copolymer product from said mixture by volatilizing theunpolymerized components away from said copolymer product.

2. The process claimed in claim 1, wherein said monovinyl aromaticmonomer is styrene.

3. The process claimed in claim 1, wherein said homogeneous monomericmaterial is composed of from about 55 to 90 percent by weight ofstyrene, from about 1 to precent by weight of alpha-methylstyrene, andfrom about 3 to 15 percent by weight of methacryclic acid.

4. The process claimed in claim 1, wherein said homogeneous monomericmaterial consists of from to 75 percent by weight of styrene, from 20 to40 percent by weight of alpha-methylstyrene, and from 6 to 10 percent byWeight of methacrylic acid.

5. The process claimed in claim 1, wherein said continuous steps are insequential order.

6. The process claimed in claim 1, wherein said monomeric material isfed into said polymerization zone at a rate to maintain thepolymerization temperature substantially constant.

7. The process claimed in claim 1, wherein the proportion of saidcopolymer product in said polymerization zone is maintained constant atfrom about 15 to percent by weight of said mixture.

8. The process claimed in claim 1, wherein said polymerization step (2)is conducted at a pressure of from about 25 to pounds per square inchgauge.

9. The process claimed in claim 1, where said copolymer product isisolated by devolatilization of said mixture.

References Cited in the file of this patent UNITED STATES PATENTS2,727,884 McDonald et al Dec. 20, 1955 2,816,890 Baer Dec. 17, 1957

1. A PROCESS FOR PREPARING THERMOPLASTIC COPOLYMER OF MOLDING GRADECOMPRISING AS CONTINUOUS STEPS (1) THE PREPARATION OF A HOMOGENEOUSMONOMERIC MATERIAL COMSISTING OF ESSENTIALLY OF FROM ABOUT 55 TO 97PERCENT OF WEIGHT OF A MONOVINYL AROMATIC MONOMER OF THE BENZENE SERIESCONTAINING ONLY HYDROGEN SUBSTITUTION ON THE VINYL GROUP AND FROM ABOUT1 TO 40 PERCENT BY WESIGHT OF SAID MONOMERIC MATERIAL OFALPHA-METHYLSTYRENE AND FROM ABOUT 2 TO 15 PERCENT BY WEIGHT OFMETHACRYCLIC ACID, (2) THE EXPOSURE IN A POLYMERZATION ZONE OF A MIXTUREOF SAID HOMOGENEOUS MONOMERIC MATERIAL AND THE COPOLYMER PRODUCT OFPOLYMERIZATION THEREOF IN THE LIQUID STATE TO A POLYMERIZATIONTEMPERATURE BETWEEN ABOUT 130*C. AND ABOUT 160*C., THE PROPORTION OFSAID COPOLYMER PRODUCT BEING NOT MORE THAN ABOUT 60 PERCENT BY WEIGH OFSAID MIXTURE, (3) THE MAINTENANCE OF THE PROPORTIONS OF SAID MONOMERICMATERIAL AND SAID COPOLYMER PRODUCT IN SAID MIXTURE SUBSTANTIALLYCONSTANT BY FEEING ADDITIONAL OF SAID MONOMERIC MATERIAL TO THE MIXTUREWHILE WITHDRAWING SAID MIXTURE OF COPOLYMER PRODUCT AND UNPOLYMERIZEDMONOMERS FROM SAID ZONE AT ABOUT THE SAME VOLUMETRIC RATE AT WHICH SAIDMONOMERIC MATERIAL IS BEING FED TO SAID MIXTURE, ALL OF SAID STEPS BEINGCARRIED OUT IN THE SUBSTANTIAL ABSENCE OF IRON, AND FINALLY (4)ISOLATING SAID COPOLYMER PRODUCT FROM SAID MIXTURE BY VOLATILIZING THEUNPOLYMERIZED COMPONENTS AWAY FROM SAID COPOLYMER PRODUCT.